It sounds like a sheared flywheel key since you're getting spark and it's backfiring. A sheared key would throw it out of time. The other thing I would check for would be a stuck valve. Do you feel compression at the spark plug hole with your thumb when the engine is turning over?
Believe it or not -- in the flywheel.
You remove the flywheel nut and washer to do an inspection. If you can let me know what engine is on there (model, type/spec, code/serial) I should be able to get a drawing to post for you. Here's some good info that may help.
There should be a model, type, and code. Those numbers are for EPA emissions.
Understood. Let me know when you would like to continue...
Here's a parts list for your engine to show you how everything goes together.